1. Field of the Invention
This invention relates to a color image processing technique for converting an input color image signal to a color image record signal of a color printer and the like, and particularly to the technique for converting a color image signal including a solid color or further black ink to a color image record signal including a solid color or further black ink along with yellow, magenta, and cyan.
2. Description of the Related Arts
In the printing technology, as a technique for representing a vivid color not reproduced by four-color process printing using yellow (Y), magenta (M), cyan (C) and black ink (K), a solid color composed of primary color ink of red (R), green (G) and blue (B) or fluorescent ink is added to four colors YMCK to perform color reproduction. As a color sample of a solid color, PANTONE formula guides is known, and about solid colors of about 1000 types are defined.
In the printing machine, however, only eight colors at maximum can be printed at the same time, so the number of solid colors to be used is as small as no more than eight colors and there have been designers' demands toward increase in number of available solid colors. Further, in the case of changing the solid color in every impression, cleaning work for the printing machine is performed, resulting in the problem that solid color printing takes a great amount of man-hour in a printing site.
Further, in the workflow of printing, a manuscript of a natural picture has been sent to a printer by a digital camera. Since the sRGB color space, which is the color space of the digital camera, is wider than the color gamut of four-color YMCK process color printing, there have been strong demands towards enlargement of the color gamut of printing aiming at the faithful reproduction.
In order to cope with the above problems and requests, in the printing technology, a color reproduction method called HiFi color has been proposed, in which in addition to the four-color YMCK process color, at least one of solid colors RGB is added for color reproduction in five to seven colors so as to enlarge the color gamut. As the HiFi color printing, PANTONE hexachrome has been widely used, and in that printing, in addition to four colors YMCK, R-series orange (O) ink and G ink are added to perform color reproduction in six colors, whereby about 90% of the above PANTONE formula guide can be reproduced. This color reproduction method of HiFi color printing (hereinafter referred to as HiFi color) has been widely applied to an ink jet method and an electrophotographic color printer as well, so it is general as a technique for enlarging the color gamut.
On the other hand, with the recent development of computer technology, network technology and color printer technology, as for the prints for which an order has been placed from a printing office heretofore, an electronic printing original document is created by a computer in an office or at home, and in the case of a small number of copies, the prints are output by a color printer of the office or home, and in the case of a large number of copies, desk top publishing (hereinafter referred to as DTP), that is, sending of a manuscript to a printing office by an electronic document has been extensively carried on. In the case of the DTP, an object of an output device is printing, so it is general to create electronic print data on a computer with image record signals in printing. For example, in the case of the hexachrome printing described above, an electronic manuscript is represented by color signals of six colors YMCKOG, in which orange and green are added to four colors YMCK: yellow, magenta, cyan and black ink.
In the case of specifying the colors of the electronic manuscript using the image record signals in printing as described above, black ink is specified in characters, graphic forms and natural drawings from the respective different point of views by an editor creating the electronic manuscript. To be concrete, the characters and graphic forms are specified by black ink color only regardless of the density of the characters and fine lines in order to improve the visibility of the black characters and black fine lines. Further, it depends on the editor's intention of presentation whether the gray of the graphic form is represented by process black ink of yellow, magenta and cyan, or represented by the black ink color only.
On the other hand, in the natural images, the black ink is ordinarily generated through inking processing such as UCR (Under Color Replacement) and GCR (Gray Component Replacement) by photo retouch software or a scanner. At the time, it is usual that in an area having a low density, inking is not performed to secure the graininess and gradient, but in an area having a high density, inking is performed. When the inking amount is large, color reproduction is deteriorated. Accordingly, in the natural images, frequently the inking rate is set low.
As described above, the black ink specifying method varies with the characters, graphic forms and natural images. Accordingly, when they are output to a printer, the same black ink amount is attained in the input and output as the editor's intention, and in a manuscript, a part specified by the black ink color only is required to be output in the black ink color only.
Similarly to the black ink, the solid colors in the HiFi color printing are also specified in the characters, the graphic forms and natural drawings from the respective different points of view by an editor creating an electronic manuscript. To be concrete, it is frequent that as to the characters and graphic forms, red characters and red fine lines, which are represented by second color of yellow and magenta in ordinary process color printing, are specified by red only which is solid color regardless of the density of the character and fine line in order to prevent the deterioration of visibility due to color aberration. As to the red representation of a graphic form, it depends on an editor's intention for presentation whether the graphic form is represented by red of second color of yellow and magenta, red only which is solid color, or third color obtained by adding red as solid color to the second color of yellow and magenta.
Also in the case of using the solid color, in the natural drawings, generally UCR processing is performed by photo retouch software or scanner to generate a solid color from three primary color signals of YMC or RGB. At the time, in the process color printing, for example, in the reproduction of red represented by second color of yellow and magenta, in order to decrease the quantity of ink to reduce the printing cost, sometimes the UCR rate of the solid color is set higher so that the quantity of red as solid color is increased and the quantity of yellow and magenta is decreased so as to reproduce the red by combination of colors inclining toward first color. On the other hand, in some cases, the UCR ratio of the solid color is set lower so that the quantity of red as solid color is decreased and the quantity of yellow and magenta is increased to reduce the red by combination of colors inclining toward second color and third color, thereby producing depth in a tone of color and gradation. As described above, as to the natural drawings, the solid color specifying method depends on the editor's intention of presentation and cost request.
As described above, while the solid color is also different in specifying method between the characters, the graphic forms and the natural drawings similarly to the black ink, in the case of outputting to the printer, the same solid color is requested both in the input and in the output as the editor's intention.
In the case of printing a large number of copies, printing is performed by color reproduction method corresponding to the color signal specified by the electronic document, whereby it is not necessary to conduct color conversion processing for the color signal on the electronic document. In the case of outputting a small number of copies by a color printer, however, the color reproducibility is largely difference between press and color printer, so it is necessary to perform color conversion processing for converting input color signals including a solid color and black ink of HiFi color printing to image record signals including a solid color and black ink of the color printer. In the case where the color reproduction method specified by the electronic manuscript is HiFi color printing such as hexachrome printing, also in the color printer, the color gamut equivalent to that of the HiFi color printing is required. Therefore, it is necessary to output in print by the color printer adapted to perform color reproduction according to the image record signals including a solid color and black ink similar to those of the HiFi color printing as input signals. In the color printer, however, the coloring materials of solid colors having the quite same characteristics as those of printing cannot be always used, so it is necessary to conduct color conversion processing to the image record signals including a solid color and black ink used in the color printer. As the related art of color conversion processing for such HiFi color, the following technique has been proposed.
At present in the color management system (hereinafter referred to as CMS) based on the specifications proposed by International Consortium (hereinafter referred to as ICC) widely spread as the industry standard, after color signal conversion from the machine dependent color space including a solid color and black ink of YMCKOG color signals serving as input color signals to color signals of machine independent L*a*b* or XYZ color space, the color signals are converted to image record signals including a solid color and black ink of the machine dependent color space of the color printer. When color conversion processing is thus conducted, the input color signals and the output image record signals are of the same in the machine independent color signal, colorimetric color reproduction can be ensured so that high-accuracy color conversion can be implemented. As this type of CMS, Color Sync carried by Mac (registered trade mark) OS of Apple Computer Corp. and ICM carried by Windows (registered trade mark) of Microsoft Corp. are typical.
Conversion from the input color signals including a solid color and black ink to the device independent color signals is publicly known, so in the color conversion processing for HiFi color in accordance with the ICC, it will suffice to implement the conversion from machine independent color signals composed of three variables of L*a*b* or XYZ color space to image record signals including a solid color and black ink.
U.S. Pat. No. 4,812,899 describes an image forming apparatus adapted to perform color reproduction in 7-color process ink of YMCKRGB. In this image forming apparatus, color conversion from RGB signals of an input device such as a scanner to YMCKRGB of an output device such as a press is determined depending on UCR of achromatic component (black ink component) and chromatic component (RGB component), and a method called Kueppers Technique has been proposed. This is the first method proposed as color conversion processing for the HiFi color, and the algorithm is simple so that it is widely used.
When the application of the Kueppers Technique to color conversion in accordance with the ICC is considered, color signals of the machine independent color space such as L*a*b* color space are converted to RGB color signals by a publicly known 3-input 3-output color converting unit, and the RGB color signals are subjected to UCR processing by achromatic component and chromatic component, thereby implementing conversion from machine independent color signals to 5-color to 7-color image record signals including a solid color and black ink.
In the Kueppers Technique, however, although color conversion processing for the HiFi color in accordance with the ICC can be implemented, at the time of conversion from the machine dependent color space including a solid color and black ink such as YMCKOG color signals as input color signals to machine independent color signals of L*a*b* or XYZ color space, the solid color and black ink information of the input color signals is not retained. Consequently, encountered is the problem that the quantity of solid color and the quantity of black ink of output in conversion to image record signals including a solid color and black ink of the color printer are different from the quantity of solid color and the quantity of black ink of the input color signals.
As the color conversion processing for the HiFi color, a method called division method has been proposed. According to this method, the color gamut of the HiFi color is divided into color sub-gamuts formed by combination of three colors or four colors, and in each color sub-gamut, three or four image record signals in the color sub-gamut are determined from the machine independent color signals by the same method of the ordinary 3-color or 4-color printer in each color sub-gamut, thereby performing the conversion from the machine independent color signals to image record signals. Japanese Published Unexamined Patent Application No. 2000-32284 proposes a method of determining a coefficient of direct lookup table (hereinafter referred to as DLUT) for conversion processing from device independent color signals to image record signals including a solid color and black ink using sub-division color gamut formed by a combination of black ink and the other two colors having a close hue. Further, Japanese Published Unexamined Patent Application No. 2001-136401 proposes a method of determining a coefficient of DLUT for conversion processing from device independent color signals to image record signals including a solid color and black ink using a sub-division color gamut formed by a combination of black ink and the other three colors.
The division method, however, has the problem that although color conversion processing for the HiFi color in accordance with the ICC can be implemented, similarly to the case of Kueppers Technique, the solid color information and black ink information of the input color signals are not retained, so that the quantity of a solid color and the quantity of black ink of output are different from those of the input color signal.